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Model O Universal 

AVOMETER 



mk n 



WORKING INSTRUCTIONS 



£S?d> 



LTD. 



AVOCET HOUSE, 91-96, VAUXHALL BRIDGE ROAD, LONDON, S.W.* 

ENGLAND 



MODEL 8 UNIVERSAL 

AVOMETER 

Mkll 



INSTRUCTIONS FOR USE 




-3&m> 



LTD, 



AVOCET HOUSE, 92-96 VAUXHALL BRIDGE ROAD, LONDON, S.W.I. 

Telephone: Victoria 3404 (12 lines) ENGLAND Telegrams: Avocet, Sowest, London 

A MEMBER OF THE METAL INDUSTRIES GROUP OF COMPANIES 



FOREWORD 



For more than a quarter of a century we have been engaged in the design and 
manufacture of " AVO " Electrical Measuring Instruments. Throughout that time 
we have consistently pioneered the design of modern multi-range instruments and 
have kept abreast of, and catered for, the requirements of the epoch-making develop- 
ments in the fields of radio and electronics. 

The success of our steadfast policy of maintaining high standards of performance 
in instruments of such wide versatility, and making such instruments available at 
reasonable cost, is reflected in the great respect and genuine goodwill which " AVO " 
products enjoy in every part of the world. 

It has been gratifying to note the very large number of instances where the satisfac- 
tion obtained from the performance of one of our instruments has led to the auto- 
matic choice of other instruments from the " AVO " range. This process, having 
continued over a long period of years, has resulted in virtual standardisation on our 
products by numerous Public Bodies, The Services, Railway Systems, and Post 
Office and Telegraph Undertakings throughout the world. 

Our designers have thereby been encouraged to ensure that new instruments or 
accessories for inclusion in the " AVO " range fit in with existing " AVO " apparatus 
and serve to extend the usefulness of instruments already in being. Thus, the user 
who standardises on " AVO " products will seldom find himself short of essential 
measuring equipment, for, by means of suitable accessories, his existing equipment 
can often be adapted to most unusual demands. 

It is with pleasure that we acknowledge that the unique position attained by 
M AVO " is due in no small measure to the co-operation of so many users who 
stimulate our Research and Development staffs from time to time with suggestions, 
criticisms, and even requests for the production of entirely new instruments or 
accessories. It is our desire to encourage and preserve this relationship between 
those who use " AVO " instruments and those who are responsible for their design 
and manufacture, and correspondence is therefore welcomed, whilst suggestions will 
receive prompt and sympathetic consideration. 




THE MODEL 8 AVOMETER MK. II 

I 



CONTENTS 



foreward 

Introduction 

Table of Ranges 

General Description 

Limits of Accuracy . . 

Design and Construction 

Range Controls 

The Movement 

Scaling 

Replacement of Internal Battery and Cell 

Movement Reverse Control 

Overload Protection 

Operation of Instrument 

Current Measurement 

Voltage Measurement 

Resistance Measurement 

Insulation Resistance Measurement 

Low Resistance Measurement 

Decibels 

Accessories 

D.C. Voltage Multiplier 

Resistance Range Extension Unit 

Transformers 

Conclusion . . 

Circuit Diagram of the Model 8 Universal AvoMeter 



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COPYRIGHT 



No information or diagrams in whole or in part may be copied or reproduced without the prior permission 

in writing of Avo Limited 



® 



INTRODUCTION 



Since its conception in 1923, the Avo Meter has maintained a distinct lead upon all 
its competitors, and can today quite rightly be termed the most popular instrument 
of its type in the world, for in no other instrument can one find such a unique combi- 
nation of ranges and comprehensive automatic overload protection, in addition to 
a high degree of accuracy, reliability and simplicity of use. 

Much time and thought is continually devoted by our design department to the 
improvement of our products and it is for the Electronic Radio and Television 
Engineer that this new instrument has been primarily produced. The Model 8 
AvoMeter Mk. II has the high D.C. voltage sensitivity of the High Resistance Avo- 
Meter Models 1 and 2, but in addition, provision is made for the measurement of 
A.C. current. A further useful feature which has been incorporated is a push button 
change-over switch which enables the direction of the current through the moving 
coil to be reversed, thus obviating the necessity of changing leads when working 
with D.C. voltages and currents which may be either positive or negative in respect 
to a basic test position. The excellent qualities of previous models including the 
" AVO " automatic cut-out have been retained, and we have great confidence that 
given a reasonable amount of care and attention, not forgetting the removal of 
exhausted batteries, this instrument will give lasting satisfaction. 



TABLE OF RANGES 



D.C. Voltage 


D.C. Current 


A.C. Voltage 


A.C. Current 


2,500 V. 


10 A. 


2,500 V. 


10 A. 


1,000 V. 


1 A. 


1,000 V. 


2-5 A. 


500 V. 








250 V. 


100 mA. 


250 V. 


1 A. 


100 V. 


10 mA. 


100 V. 


100 mA. 


25 V. 


1 mA. 


25 V. 




10 V. 


250^A. 


10 V. 




2-5 V. 


50jJ.A. 


2-5 V. 





Resistance 

— 200 megohms — with external voltage 

0-20 „ I 

0—200,000 ohms > self-contained 

0—2,000 ohms J 

— 2-5 ohms with external unit. 



• 



• 



The 

Model 8 Universal AvoMeter 
Mk II 



WORKING INSTRUCTIONS 



General Description 

The meter is supplied complete with a pair of special rubber-covered leads which 
are intended for attachment to the AvoMeter by means of its non-loss terminals. 
The remote ends of the leads are fitted with spring clips, which may be interchanged 
with the " AVO " PRODCLIPS which are supplied with the instrument. 

M AVO " Prodclips have been introduced to enable connections for test purposes 
to be made at what are normally inaccessible points on a chassis. Examination will 
show that they are completely insulated with the exception of the jaws at one end, 
which can be opened by compressing the stem into the body of the clip. Rigid con- 
nections to wiring can thus be made by this insulated device in complicated wiring 
systems where other types of larger clip could not be attached, or if fixed might cause 
short circuits. 

All tests, except those on the 2,500V. ranges, make use of the pair of terminals at 
the base of the instrument. 

The meter is extremely simple to use, range selection in general being accomplished 
by means of two switch knobs. 

A clearly marked 5" scale has uniformly divided graduations to match 100 and 250 
scale markings, and in addition there is an ohms scale and one for decibels. An 
anti-parallax mirror permits readings of the knife edge pointer to be made with great 
precision. 

Limits of Accuracy 

Generally speaking, the highest percentage accuracy on current and voltage ranges 
is obtainable at the upper end of the scale, but on resistance ranges it is better towards 
the centre of the scale. In the case of voltage measurements, which are more 
frequently taken than those of current, successive ranges have been closely chosen to 
obviate the need for taking readings on very small deflections. 



The instrument will produce its highest accuracy when used face upwards, in 
which position it has been calibrated. 

D.C. Voltage, 2% of indication between full-scale and half-scale deflection. 

Below half-scale deflection, 1 % of the full-scale value. 

D.C, Current. 1 % of full-scale value over effective range. 

A.C, Current and Voltage, (25-2000 c/s.) 2-25 % of full-scale value over effective 

range. 

The definition of" effective range " set down in the British Standard Specification 

89/1954 is as follows, when related to the AvoMeter: — 

D.C. — from 0-1 of scale-range to full-scale value. 

A.C. — from 0-25 of scale-range to full-scale value. 

It will be noted that with the exception of the D.C. voltage ranges, the instrument 
meets the requirements laid down in Section 6 of the British Standard Specification 
89/1954 for 5" (127mm.) scale-length Industrial Portable Instruments. In practice, 
the Model 8 is well within the above limits, due to the great care taken in the manu- 
facture of its various components, and to the fine initial calibration. 

Inasmuch as rectifier moving coil instruments give readings on "A.C." proportional 
to the mean and not the R.M.S. value of the wave form with which they are presented, 
they depend for their accuracy not only upon their initial calibration, but also upon 
the maintenance of a sinusoidal wave form. Since the form factor (R.M.S. value 
divided by mean value) of a sine wave is 1-11, this has been taken into account in 
calibrating the meter which does, therefore, indicate R.M.S. values on the assumption 
that the normal sine wave will be encountered. Generally speaking, considerable 
wave form distortion can occur without appreciably affecting the form factor and 
resulting accuracy of measurement, but the user should recognise the possibility of some 
error when using distorted wave forms, squarish wave shapes producing high readings, 
and peaky ones, low readings. 

Design and Construction 

The instrument consists of a moulded panel on the inside of which are mounted the 
whole of the switching apparatus, resistances, shunts, transformer, rectifier, etc., 
together with the movement. The panel fits into a robust moulded case, the joint 
being rendered completely dust proof, whilst a carrying strap is provided to facilitate 
portability. The main switching is accomplished automatically by means of two 
knobs which indicate on the engraved panel, the range in use. These switches are 
of generous and robust design, the contacts being arranged to ** make " before 
** break " on adjacent ranges; a feature which provides a factor of safety in use. 

When the instrument is set for operation on D.C, the moving coil is associated 
with a universal shunt and series multipliers, whilst on A.C, a full-wave rectifier and 
transformer are also introduced. 

Range Controls 

The left-hand knob provides all the D.C current and voltage ranges (except 2,500V.) 
and the right-hand knob the A.C. ranges (except 2,500V.) and also the resistance 
ranges. These knobs are electrically interlocked so that D.C readings can only be 
made after the right-hand switch has been set to D.C, and the left-hand switch to 
the range selected. ./l.C. readings call for the left-hand switch to be set for A.C 
(it must not be left at RESISTANCE) and the right-hand switch at the range required. 
Resistance tests require the left-hand switch to be set to RESISTANCE and the right- 
hand one to the desired range. 

8 



* 



If the switches are inadvertently left to actual ranges simultaneously, there is no 
circuit through the meter, and it is thereby safeguarded against accidental damage 
or misleading readings. 

It is possible to determine whether a source is A.C. or D.C., since A.C. will not 
produce pointer indication when the meter is set for D.C. measurement. A small 
pointer indication, however, may result if D.C. current is passed through an A.C. 
range, but no harm can be done to the meter provided it is not at the same time 
grossly overloaded. 

The main ranges are engraved on the panel around the switches, and arrow heads 
on the knobs indicate the actual range selected. In the case of voltage, successive 
ranges are built up on the ratios of 2 : 1, 2-5 : 1 and 4:1, but in the case of current, 
a wide coverage has been chosen instead and the 10:1 ratio in general is followed. 
The 2,500V. ranges A.C. and D.C. are available by means of the two special terminals 
so marked. 

Extremely wide coverage in resistance has been achieved by having a fundamental 
range as marked on the scale, together with ranges of x 100 and -~ 100 to supplement 
it. Before carrying out resistance tests, the meter should be adjusted for the state of 
the batteries. It is merely necessary to join the leads together and adjust to zero in 
the following sequence: ohms X 1 ; ohms -r 100, followed by ohms x 100, using 
in each case the adjuster to match the range. 

In addition, a 200-megohm range marked " INS " is available, using an external 
D.C. voltage source. 

The Movement 

The moving coil consists of an aluminium former wound with copper wire and 
supplemented with Constantan in order to reduce temperature error. It is pivoted 
on hardened and highly polished steel pivots between conical spring-loaded jewels, 
and swings in a gap energised by two powerfully magnetised and aged ' Alcomax ' 
blocks associated with mild steel pole pieces. Two phosphor bronze hair springs are 
fitted for the purpose of conveying current to the moving coil, and to provide control- 
ling torque. A knife edge type of pointer is fitted enabling very fine readings to be 
taken, whilst the whole movement is perfectly balanced and reasonably damped so 
that the pointer quickly comes to rest. 

Scaling 

The scale plate has three main sets of markings, each of approximately 5" length, 
the outermost being for resistance measurement and is marked 0-200,000 ohms. 
The second is for current and voltage (both A.C. and D.C.) and is marked 0-100, 
with divisions approximately 1| mm, apart. The third scale, calibrated 0-250, has 
50 divisions, and is so used for current and voltage measurements. In addition, there 
is a decibel scale marked from —15 dB. to +15 dB., which can be used with any of 
the A. Granges, 

Replacement of Internal Battery and Cell 

Inside the cover, under the carrying strap is mounted a 15V. battery and a HV. 
cell. These batteries should be examined from time to time to ensure that the 
electrolyte is not leaking and damaging the instrument. This condition will generally 
occur only when the cells are nearly exhausted. If it is known that the meter is going 
to stand unused for several months, it is preferable that these batteries should be 
removed to prevent possible damage. 



When replacing batteries, the UV. cell and the 15V. battery must be inserted with 
the poles to match the markings of polarity inside the battery box. 

Replacements : 1 -5 V. cell, 1 1" dia. X 2f ", such as Ever Ready (or overseas, Berec) U.2. 
15V. battery, 1jfe*xf*xl£*, such as Ever Ready R.121. 

Movement Reverse Control 

It sometimes happens that D.C. voltages may be required both positive and negative 
to a reference point, or the direction of flow may be reversed. In order to simplify 
the matter of lead alteration, a movement reverse press button (REV. M.C.) is 
provided. It should bs noted that the polarity marked on the terminals is for normal 
use and does not apply when the button is pressed. 

Overload Protection 

Apart from the ability to do its job, one of the most attractive features of the 
instrument is the provision of an automatic cut-out which gives a very high degree of 
overload protection to the whole of the instrument. The incorporation of this device 
will be found to be of particular value when conducting experimental work, for it 
imparts to the user the feeling of mental ease and confidence. When conducting 
experimental work with conventional moving coil meters, these can be easily ruined 
by inadvertently applied overloads, whereas the AvoMeter is so well protected that 
it can withstand considerable mishandling. 

If an overload is applied to the meter, the cut-out knob springs from its normal 
position in the panel, thus breaking the main circuit, and this knob has only to be 
depressed to render the instrument again ready for use. It is important to note that 
the cut-out should never be reset when the instrument is connected to an external 
circuit, whilst the fault which caused the overload should be rectified before the meter 
is reconnected. 

The mechanism is brought into operation by the moving coil coming into contact 
with a trigger just beyond its full-scale position. There is, in addition, a second 
release at the zero end, so that the cut-out is tripped if the meter is overloaded in reverse. 

Although the overload mechanism gives almost complete protection to the meter, 
it cannot be guaranteed to fulfil completely its function in the very worst cases of 
misuse, such as the mains being connected across the meter when set to a current 
range. It should be noted that mechanical shock to the instrument will sometimes 
trip the cut-out mechanism. The cut-out should be reset, using direct pressure and 
without twisting the button, the instrument lying face upwards. 

WARNING 

Special care must be taken when using the instrument to service television receivers 
or other apparatus employing condensers of large capacity, for the inclusion of such 
components in a circuit may mean that very heavy peak currents may flow when the 
apparatus is switched on. Such surges produce a peaky waveform, and although these 
peaks are of only a few milliseconds duration, they may, never-the-less, puncture the 
instrument rectifier. It is impossible to guard against this cause of damage by means 
of any form of cut-out mechanism, but instruments manufactured since July 1 954 have 
been fitted with a surge suppression rectifier (SAI) across the main rectifier, to give the 
maximum protection which can be devised. 

10 



OPERATION OF INSTRUMENT 

The meter is intended for use horizontally. Should it happen by any chance that 
the pointer is not on zero, it may be so set by means of the screw head on the panel. 

The leads fitted with prods or clips, as required, should be connected to the lower 
pair of meter terminals in all cases except when measuring voltages over 1,000V. 
(see next paragraph). 

When measuring current or voltage, ensure that the instrument is set to match the 
type of source to be measured (either A.C. or D.C.) and then choose a suitable range 
before connecting up to the circuit under test. When in reasonable doubt, always 
switch to the highest range and work downwards, there being no necessity to disconnect 
the leads as the switch position is changed. Do not, however, switch off by rotating 
either of the knobs to a blank position. If the voltage should exceed 1 ,000V., the 
instrument should be set to measure 1,000V. as described above, but the negative 
lead should be transferred to the appropriate 2,500V. terminal. 

The instrument is flash tested at 6,000V. A.C., but should the meter be used with 
accessories on circuits in excess of 2,500V., it should be kept at the low potential end 
of the circuit (near earth potential). Tf this procedure cannot be adopted other 
suitable safeguards must be applied. 

CURRENT MEASUREMENT 

To measure current, the instrument should be set to a suitable A.C. or D.C. range, 
and then connected in series with the apparatus to be tested. 

Generally speaking, the power absorbed in the instrument is negligible, but in cases 
of low voltage heavy current circuits, the inclusion of a meter may reduce the current 
appreciably below the value which would otherwise prevail. The potential drop at 
the meter terminals is in the order of £V, on all D.C. ranges, except the 50 microamp 
range which has a drop of 125 milli-volts. In the case of A.C, it is less than |V. on 
all ranges. 

Standard meter leads have a resistance of 0-02 ohms per pair. 

In certain cases, care should be taken to ensure that the circuit is " dead " before 
breaking into it to make current measurements. 

VOLTAGE MEASUREMENTS 

When measuring voltage, it is necessary to set the appropriate range of" A.C." or 
" D.C." and connect the leads across the source of voltage to be measured. If the 
expected magnitude of the voltage is within the range of the meter, but its actual 
value is unknown, set the instrument to its highest range, connect up and if below 
1,000V. rotate the appropriate selector switch, decreasing the ranges step by step, 
until the most suitable range has been selected. Great care must be exercised when 
making connections to a live circuit, and the procedure should be entirely avoided if 
possible. 

On D.C. ranges, the meter consumes only 50 microamps at full scale deflection, 
this sensitivity corresponding to 20,000 ohms per volt. In the case of A.C. ranges 
from 100V. upwards, full scale deflection is obtained with a consumption of 1mA. 
(1,000 ohms per volt). The 25V., 10V., and 2-5V. A.C. ranges consume 4, 10 and 

11 



40mA. respectively at full scale deflection. The meter maintains a high degree of 
accuracy for audio frequency tests up to 10 kc/s on ranges up to 250V. A.C. 

Whilst discussing the problem of measuring voltage, it would be well to draw 
attention to the fact that in certain circuits where the current is limited because of the 
presence of a resistance between the scource and the point at which a measurement 
is to be made, it is possible for the actual voltage to be higher normally than when the 
meter is connected. All current consuming voltmeters, however sensitive, draw 
current to varying degrees from the circuit under test, thus causing a higher volts 
drop in the resistances mentioned, and thereby causing the voltage to fall at the point 
of measurement. , 

Owing to the high sensitivity of the Model 8 on its D.C. ranges, this effect is unlikely 
to be of importance except in a very few instances. A practical example of where it 
might be taken into account is in the measurement of EHT voltage on a television 
set or the tapping on a potential divider, where the resistances are so high as to be 
comparable with the resistance of the meter on the range in use. It is generally 
possible to use a meter on a higher range than absolutely necessary, and in such a 
case the higher meter resistance causes less disturbance than would otherwise be the 
case. At the same time adequate pointer deflection for reasonable accuracy should 

be attained. 

When it is essential to obtain an accurate indication of the voltage developed across 
a high resistance it is sometimes preferable to insert the meter in series with it, and to 
measure the current flowing. The reading given upon the meter, in milhamps, multi- 
plied by the value of the resistance in thousands of ohms, will give the developed 
voltage. 

RESISTANCE MEASUREMENT 

There are three self-contained ranges covering from 0-5 ohms to 20 megohms, and 
provision is also made for both upward and downward extension of these limits. 
The self-contained ranges make use of the usual series circuit, and successive ranges 
are on 100 : 1 ratio, which permits of very wide coverage with three ranges. 

On resistance ranges, the meter must not merely start from its normal instrument 
zero but must have, in addition, a resistance zero corresponding to the full scale 
deflection of the meter. Before carrying out tests for resistance a check and, if 
necessary adjustment should be carried out to ensure that when the leads are joined 
together the meter actually indicates zero ohms, irrespective of the condition of the 
battery (within the limits of adjustment). The method of adjustment is described later. 

Owing to the nature of the scale, it is not easy to define the accuracy, but it should 
be within 3°/ of the reading about centre scale, increasing up to about 10% of the 
indication around deflections corresponding to 10% and 90% of full scale deflection. 

Resistance test should never be carried out on components which are already 
carrying current. 

On three ranges which utilise the internal source of voltage, it should be remem- 
bered that a positive potential appears at the negative terminal of the instrument 
when set for resistance tests. This fact may be important because the resistance of 
some components varies according to the direction of the current through them, and 
readings therefore, depend upon the direction in which the test voltage is applied, 
quite apart from its magnitude. Such cases include electrolytic condensers and rectifiers. 

When measuring the leakage resistance of an electrolytic condenser, the negative 
lead from the meter should be connected to the positive terminal of the condenser, 
and the ohmsx 100 range employed. 



12 



Before making resistance tests the pointer should be adjusted to zero in the following 
sequence: — 

1. Set left-hand switch at M RESISTANCE ". 

2. Join leads together. jk-tco^o" 
3 On the Q range, adjust to zero by means of the knob marked ZfcKU u . 

4. On the " G -r 100 " range, adjust to zero by means of the knob marked 
m 7PRO Q — 100 ". 

5. On the "Q X 100" range, adjust to zero by means of the knob marked 
' "ZERO fix 100". 

To test a resistance, set the right-hand switch at the range required, the leads being 
connected across the unknown component. 

Resistance is read directly on the " Q" range, but indications should be divided 
or multiplied by 100 on the other two ranges. 

If on joining the leads together it is impossible to obtain zero ohms setting, or if 
furthermore the pointer position will not remain constant, but falls steadily, the 
internal battery or celt concerned should be replaced. It is important that a discharged 
unit should not be left in the instrument, since the electrolyte might seep through 
and cause damage to the meter. 

NOTE It can so happen that a 15-volt battery may age in such a manner that 
although it indicates a potential of 15 volts, its internal resistance has increased so 
much that some loss of accuracy can occur on the high resistance range (fix 100 • 
If the battery has been in use for some time, or if errors are suspected on the high 
resistance range, it is worth while removing the battery and checking its short circuit 
current on the 100mA. D.C. range. If the battery fails to give a reading greater than 
5mA. it should be discarded. 

INSULATION RESISTANCE MEASUREMENT 

Two courses are open, the first merely calling for a battery or other source of 
D C voltage in the order of 130V. to 160V. The left-hand switch should be set at 
" RESISTANCE " with the right-hand switch at " INS " and the meter leads should 
be connected to the battery. The pointer should be brought to zero on the ohms 
scale by means of the adjuster marked - ZERO Q X 100". To test connect the 
unknown resistance in series with the meter and its value will be that shown on the 
ohms scale multiplied by 1,000. Resistances up to 200 megohms can, therefore, be 
read on this range. 

The alternative method makes use of the " Model 8 Resistance Range Extension 
Unit," described later. 

LOW RESISTANCE MEASUREMENT 

The meter setting marked L.R. is for use with the Model 8 Resistance Range 
Extension Unit. The method of use is described in the section upon accessories. 

DECIBELS 

The decibel scale can be used with any of the A.C. current or voltage ranges. It 
has a logarithmic scale shape and is useful in so far that it gives a measurement 
closely related to the impression of aural intensity in sound reproduction apparatus 
A difference of one decibel is about the minimum difference which can be appreciated 
when comparing two intensities. For convenience, the scale is marked in decibels 

13 



both positive and negative from a reference point. The difference in level between a 
negative value on the dB. scale and a positive one is the sum of the two, i.e. the differ- 
ence between - 5 dB. and +6 dB. is 5 + 6 = 11 dB. 

It will be appreciated that when changing from one current or voltage range to 
the next higher, the pointer indication will fall, although input is kept constant. For 
a current or voltage range ratio of 2£ : 1 this corresponds to a reduction of 8 in the 
indication on the dB. scale. It follows, therefore, that 8 should be added to the 
reading every time an increase of 2 1 times takes place on the range. In the same way, 
12 should be added for an increase of 4 times on the range, or 8 + 12 = 20 dB. for 
an increase of 2\ X 4 = 10 times in the range ratio. 

The following might serve as an example: Suppose that the meter is connected on 
the 25 V. A.C, range across the primary of an output transformer and that a reading 
of +9 dB. is indicated (corresponding to 12-5V. on this range). If now the output 
increases to say 40 volts, necessitating a change to the 100V. A.C. range, the pointer 
will indicate +7 on the dB. scale. 

The 4 ; 1 increase in the voltage range calls for an addition of 12 to the dB. indica- 
tion, so that its true value represents +19 dB. The increase over the original reading 
is 19-9 = 10 dB. 

ACCESSORIES 

D.C. Voltage Multipliers 

\§kV D.C. Multiplier 

A lOkV. D.C. Multiplier has been developed mainly to enable tests to be carried 
out in television circuits. The multiplier should be connected in series with the meter 
on its 2,500V. D.C. range, in which state maximum consumption on measurement 
cannot exceed 50 microamps, and may be considerably less. It is recommended 
that the meter is kept as near earth potential as possible, and the multiplier used at 
the high potential end, e.g., when measuring an E.H.T. voltage where the negative 
line is earthy, the multiplier should be connected between the point of positive 
potential and the positive terminal of the meter, the negative lead being connected 
to the terminal marked 2,500V. D.C.—. We do not recommend, in such cases, 
connecting the multiplier to the 2,500V. D.C— terminal and pressing the moving coil 
reverse button, notwithstanding the fact that the meter is at the earthy end of the 
circuit. 

25kV D.C. Multiplier 

A 25k V. D.C. Multiplier is available for use in series with the meter set to its 10V. 
D.C. range, readings being made direct in kV on the 0-25 scale. It is most important 
to ensure that the meter is kept in the earthy end of the circuit and the multiplier 
connected to either the positive or negative terminal whichever is at high potential. 
This method of connection to get forward pointer indication with the meter earthy 
is recommended as we do not think it desirable to use the moving coil reverse button 
when measuring high voltage. 

In general we recommend that neither the meter, multiplier nor leads are handled 
whilst high voltage tests are in progress, and a special lead is provided with the 
multiplier for connection to the high potential point. 

NOTE. The 2,500V D.C. range is not employed when using this multiplier. 

14 



Resistance Range Extension Unit 

This device enables the meter to be used for both high and low resistance measure- 
ments. It is complete with batteries (except in some instances) and switching to 
facilitate tests. The device should be connected to the lower terminals on the meter. 

For high resistance the meter is set to the M Q X 100 " position, the Unit switch 
at the " SET " position and the unknown resistance should then be connected to the 
" High " terminals. Adjustment to full scale deflection should be performed by 
means of the " ZERO fi X 100 " knob. The Unit switch should then be rotated to 
" TEST ", and the reading on the ohms scale noted. Its value is that shown multi- 
plied by 1,000 corresponding to a range of 200 megohms. 

On the low range the Unit switch should be placed at " SET ", the unknown 
resistance connected to the " Low " terminals and adjustment to full scale deflection 
cirried out by means of the " ZERO Q " knob. The Unit switch should then be 
moved to position marked " TEST " and the pointer deflection on a uniformly 
divided scale noted. Full scale deflection corresponds to 2\ ohms. 

In order to avoid discharging the batteries, immediately tests have been completed 
the test leads and resistance should be removed from the unit, its switch set to the 
" low set " position, and the unit disconnected from the meter. 

Replacements: 1 -5V. Cell - 1 §* dia. X 2§", such as Ever Ready (or overseas, Berec) U.2 
Four 30V. batteries -l^xfx 2ft", such as Ever Ready B.123. 

Transformers 

Transformers as used on the Model 7 AvoMeter are equally applicable for use with 
the Model 8, when set to 100mA. A.C. It is necessary to connect the meter up to the 
secondary of the transformer before current is passed through the primary, and care 
should be taken that the cut-out is in position. If this course is not followed, quite a 
considerable voltage will appear at the secondary terminals, if current passes through 
the primary. Transformers for 50 amp., 100 amp., 200 amp. and 400 amp. are 
available. 

CONCLUSION 

Due to the high operational standards maintained throughout our organisation, 
and the close limits within which we work, breakdowns are comparatively rare, and 
can often be traced to transit damage or careless handling, for which the Company 
cannot be held responsible. Should you at any time have to return your instrument 
to the Company for repair, pack it carefully and enclose a note informing our engineers 
of the faults which vou have found, 

E. & O. E. 




15 



f 



L.R..A-HOO 
AND H 



A.C. SWITCH CAM. 
OUTER CAM . 

abOPEN 

COPEN 

deOPEN 

ALU fab CLOSED 

A.C. RANGES « C CLOSED 

\ de CLOSED 

ALL OTHER i^gLOSED 

POSITIONS j^g™ 

I INNER CAM. 

■|NS. fl X,^{f»OP|N 

ALL OTHER/ fa CLOSED 
POSITIONS I hj OPEN 







D.C. SWITCH CAM. 



Ad 



RESISTANCE 

. POSITION 



op. CLOSED 
k I. CLOSED 

{ mnCLOSED 



ALL OTHER 



W OPEN 

mn OPEN 
op CLOSED 



A C f "p. OPEN 

POSITION{ ttl.OPEN 

I mn.OPEN 






3 



REV M/C 



m/c. 37-5uA. f.sd, 3333 R 

WITH SWAMP. 



Q2SOO V. 
i A.C. 



/ ■ LB. I 250V. \ 2SV. \ 25V 
SJ-HOO IOOOV. IOOV. IOV. 



/ IA. / IOA. 
JOOmA, 2SA 



A.C. SWITCH 




/ 
IOA. 



I A. / lOtnA / 2SO 50 
lOOtnA. tmA. yuA. /jA 



V^ 



v 

D.C. SWITCH 




D.C. + 4 A.C. COMMON. D.C.-i AC, SWITCH RANGES. 

CIRCUIT DIAGRAM OF THE MODEL 8 AVOMETER 



I by Ci catty * of Hartford lid 



Pari No 100/2-265